The invention relates to new, synthetic swelling clay minerals, as well as to a process for the preparation of such clay minerals.
Clay minerals are solid substances, substantially made up of metal and oxygen atoms, whose crystal lattice has a layered structure. This layered structure consists of three repeating layers. Located centrally in this elementary three-layer structure is a layer of substantially trivalent or substantially divalent metal ions (cations). Examples of clay minerals with substantially trivalent ions are montmorillonite and beidellite; examples of clay minerals with substantially divalent ions are hectorite and saponite. The metal ions present in the central layer are octahedrally surrounded by oxygen and hydroxyl ions. In a clay mineral with trivalent ions, two of the three octahedron positions are occupied by metal ions. Accordingly, this is referred to as a di-octahedral clay mineral. In a clay mineral with divalent metal ions, all three octahedron positions are occupied by metal ions; this is referred to as a tri-octahedral clay mineral. On opposite sides of this layer of octahedrally surrounded metal ions occurs a layer of tetrahedrally surrounded ions. These tetrahedrally surrounded ions are generally silicon ions, while a part of the silicon can optionally be replaced by germanium. The unit of the tetrahedrally surrounded silicon ions is Si.sub.2 O.sub.5 (OH). In this connection it is noted that in the tetrahedron and octahedron layers the actual point where the charge is located cannot always be indicated equally clearly. The term 'ions' as used in this context accordingly relates to the situation where an atom, given a completely ionic structure, should possess an electrostatic charge corresponding with the oxidation state.
Essential to clay minerals is that a part of the cations present are substituted by ions of a lower valency. Thus it is possible to substitute a part of the trivalent or divalent metal ions in the octahedron layer by divalent and monovalent metal ions, respectively. With substantially trivalent metal ions, this substitution results in montmorillonite and with substantially divalent metal ions in hectorite. It is also possible to substitute the tetravalent silicon ions in the tetrahedron layers by trivalent aluminum ions. With a clay mineral with almost exclusively trivalent ions in the octahedron layer, the result is then a beidellite and with a clay mineral having almost exclusively divalent ions in the octahedron layer, the result is a saponite. Of course, substitution by an ion of lower valency leads to a deficiency of positive charge of the platelets. This deficiency of positive charge is compensated by including cations between the platelets. Generally, these cations are included in hydrated form, which leads to the swelling of the clay. The distances between the three-layer platelets is increased by the inclusion of the hydrated cations. This capacity to swell by incorporating hydrated cations is characteristic of clay minerals.
If no metal ions or silicon ions are substituted by ions of a lower valency, the platelets are not charged. The mineral then does not absorb any water into the interlayer and therefore does not swell. The mineral with exclusively aluminum in the octahedron layer and silicon in the tetrahedron layer is pyrophyllite and the mineral with exclusively magnesium in the octahedron layer and silicon in the tetrahedron layer is talc. The swelling clay minerals having a negative charge of from 0.2 to 0.6 per unit cell, -O.sub.10 (OH).sub.2, are known as smectites.
The cations in the interlayer of swollen clay minerals are strongly hydrated. As a result, these ions are mobile and can be readily exchanged. The exchange is carried out by suspending the clay mineral in a concentrated solution of the cation to be provided in the interlayer. The high concentration provides for a concentration gradient as a result of which the exchange proceeds. Upon completion of the exchange, the concentrated solution is removed by filtration or, preferably, by centrifugation and washing, whereafter, if necessary, the last metal ions not bound in the interlayer can be removed by dialysis.
The negative charge of the platelets can be compensated not only with hydrated cations, but also with (hydrated) hydrogen ions, H.sub.3 O.sup.+. In this case the clay can function as a solid acid, which leads to important catalytic applications. Suspending a clay mineral in a concentrated acid does not lead without more to the provision of hydrogen ions in the interlayer. In fact, it has been found that the acid reacts with the cations of the clay structure, so that these ions are removed from the clay structure. These cations eventually end up in interlayer positions.
If it is desired to provide Br.o slashed.nsted-acid groups in a hydrated clay mineral, in general hydrolysing metal ions are provided in the interlayer. As a result of the hydrolysis, hydrogen ions are formed. Upon reduction of the amount of water in the interlayer, for instance through thermal desorption, the acid strength increases. Due to the lesser amount of water, the residual water molecules are polarised more strongly by the metal ions. Upon complete removal of the water, however, the Br.o slashed.nsted-acid groups disappear. If it is desired to impart Br.o slashed.nsted-acid properties to clay minerals at elevated temperatures, (hydrated) ammonium ions can be provided in the interlayer. Upon heating, the water and the ammonia escape while a proton remains behind.
Natural clay minerals have long been used for the practice of catalytic reactions in liquid and in gaseous phase. In general, the catalytic activity of clay minerals is based on the presence of Br.o slashed.nsted- or Lewis-acid groups in the clay minerals. In the conventional acid-catalysed reactions in the liquid phase often sulfuric acid is used. This acid yields Br.o slashed.nsted-acid groups while, moreover, it can dehydrate in that it has strong water-binding properties, and can take up undesired higher molecular by-products. What results, however, are large amounts of polluted sulfuric acid, acid tar, for which it is difficult to find any use. Neutralisation of large amounts of sulfuric acid used as catalyst leads to ammonium sulfate, which can be disposed of as less high-grade fertilizer, which is useful only for a business which also produces and/or sells other kinds of fertilizer.
In syntheses where Lewis-acid catalysts are needed, such as the Friedel-Crafts synthesis, metal chlorides, such as aluminum chloride, are used as catalyst. Hydrolysis of the aluminum chloride upon completion of the reaction leads to large amounts of highly corrosive suspensions of aluminum hydroxide.
Accordingly, both the use of sulfuric acid and the use of Lewis-acid catalysts, such as aluminum chloride or zinc chloride, entail drawbacks. Therefore, there is a need for solid acid catalysts that are suitable for carrying out such acid-catalysed reactions. Accordingly, one of the objects of the invention is to provide such solid acid catalysts for carrying out reactions in the liquid and/or gaseous phase, which are catalysed by Br.o slashed.nsted- and/or Lewis-acids.
Of great importance in this connection is the degree of hydration of the clay minerals. If water-immiscible, liquid reactants are to be processed, the presence of water on the surface of clay minerals prevents the required intensive contact between the reactants and the clay surface. The water will preferentially wet the clay surface. In many liquid phase reactions, therefore, it will be necessary to priorly dehydrate the clay mineral to be used. This must take place without any substantial reduction of the accessible clay surface. Also, the reagents used will generally have to be dried to a far-reaching extent.
Another important problem with the use of solid catalysts in liquid phase reactions is the separation of the catalyst from the reaction mixture. Generally, this is effected by filtration or centrifugation. The known, mostly natural, clay minerals generally lead to a compressible filter cake. This makes it cumbersome to separate the clay mineral by filtration or centrifugation from the reaction products and unreacted reactants. One of the tasks of the invention is therefore to provide clay minerals in a form which is readily separable from the reaction products and unreacted reactants.
Another problem occurring in catalytic reactions in the presence of heterogeneous catalysts relates to the occurrence of transport impediments in the porous catalyst body. In the liquid phase diffusion coefficients are generally a factor 10.sup.4 lower than in the gaseous phase. As a result, soon transport impediments arise when high-porosity solid catalysts are used in liquid phase reactions. Especially in organo-chemical reactions transport impediments have a highly adverse effect on the selectivity. Thus, it will be desired, when alkylating benzene for instance, to minimize the amounts of di- or tri-substituted reaction products. This is possible only when reactants and reaction products are quickly transported through the solid catalyst. This requires a catalyst with short and wide pores. A third task of the invention is thus to provide clay minerals with short, wide pores which can be readily separated from a liquid phase.
In summary, clay minerals for use as liquid phase catalysts should satisfy the following, partly contradictory, requirements:
(i) a far-reaching dehydration must be possible without a substantial reduction of the active surface accessible to reactants, PA1 (ii) ready separation of the liquid phase in which the reaction has been carried out, PA1 (iii) excellent transport properties, that is, the presence of wide, short pores; short pores require small catalyst bodies, which renders the separation from the liquid phase more difficult again.
In the gaseous phase clay minerals were used especially for catalytically cracking petroleum fractions. By the end of the thirties, natural clays were used on a large scale in the catalytic cracking of petroleum fractions. Soon, however, clay minerals were replaced by amorphous aluminum oxide-silicon dioxide catalysts, which were found to satisfy better the requirements of the technical implementation of the cracking process. By spray-drying, on the basis of amorphous aluminum oxide-silicon dioxide, wear-resistant bodies of dimensions of from 50 to 200 .mu.m could be easily produced. These bodies are simple to transport in a gas stream from the regeneration zone to the cracking zone.
Subsequently, in the sixties, cracking catalysts based on zeolites were developed, which exhibited a higher activity and selectivity. From natural clays, bodies of the required dimensions can be prepared which contain only zeolite crystallites. In general, however, small zeolite crystallites (approximately 1 .mu.m or less) are included in amorphous aluminum oxide-silicon dioxide, which functions as binder. The limited dimensions of the pores in zeolites have as a consequence that heavier fractions can no longer be cracked with zeolites.
The elementary platelets of current, natural clay minerals are relatively large, &gt;1 to 30 .mu.m, while mostly a large number of platelets, viz. more than 20 to 50 elementary platelets, are stacked into packages. As a result, upon dehydration, which renders the interlayer inaccessible, the catalytically active surface is relatively small. Is has now been found that the accessible surface of dehydrated clay minerals can be markedly enlarged by 'pillaring' the clay mineral. In that case, by metal ion-exchange hydrated oligomers or polymers of inter alia aluminum, zirconium, titanium and/or chromium are provided between the clay layers. Upon dehydration, a metal oxide 'pillar' is left. After dehydration, the distance between the clay layers varies from 0.6 to 1.6 nm. It is endeavored to realise even greater distances between the clay layers by arranging greater pillars. This is to make it possible to process heavier petroleum fractions.
Especially around 1980, much research was done on the pillars of clay minerals, as appears from the number of patent applications filed and the number of patents granted. An example is U.S. Pat. No. 4,176,090, which discloses pillared clay materials that are useful as catalysts and sorbents. According to this patent specification, an aqueous suspension of a natural clay mineral, such as calcium bentonite or beidellite, is prepared and the suspension is mixed with a solution of polymeric metal (hydr)oxide particles. The positively charged polymeric complexes exchange with the cations originally present in the clay. Then the clay is separated from the aqueous solution, the material is dried, and finally calcination is carried out at a temperature of about 200 to 700.degree. C. While initially the interlayer is completely filled with water, in which the originally present cations or the polymeric complexes occur, after drying and calcination only the oxide of the polymeric complex is present. The greater part of the interlayer is now accessible to gas molecules since the elementary clay layers are kept separate by the oxide formed from the polymeric complex. As appears from the examples included in the patent specification, half an hour is sufficient to complete the exchange in the aqueous suspension. It should be noted here that in the examples relatively small clay particles are used, viz. less than about 2 .mu.m. This appears from Example 3 of the above U.S. Pat. No. 4,176,090; in fact, it is communicated that the separation of the clay particles from the aqueous phase poses problems. For this reason, in that example a flocculating agent is used. For this purpose, inter alia a sodium silicate solution can be used.
Mentioned as pillaring agents are positively charged hydroxy complexes of aluminum, zirconium, and/or titanium. In one of these examples, a mixed hydroxy complex of magnesium and aluminum is prepared. In most examples of the above U.S. Pat. No. 4,176,090 pillaring is carried out with polymers based on hydrated aluminum oxide. It is possible to prepare discrete complexes with thirteen aluminum ions, the so-called Al.sub.13 complex. However, it is difficult to obtain this complex in pure form; nearly always a considerable part of the aluminum is present in the system in a different form, while the strongly diluted solutions of Al.sub.13 generally necessitate large volumes of water. For the preparation of pillared clay minerals on a technical scale, this is a disadvantage.
The distance of the elementary platelets in the clay structure, which is easy to determine by X-ray diffraction, is 0.7 to 1.0 nm after pillaring and after calcination. The BET surface varies from 150 to 600 m.sup.2 per gram and the pore volume from 0.1 to 0.6 ml per gram. Further, it is found that more than 50% of the surface and in many cases even more than 75% of the surface is present in pores of a size less than 3 nm. This means that the elementary platelets of the clay structure are stacked to a considerable extent. If the elementary platelets were arranged relatively arbitrarily, as in a house of cards, a much larger fraction of the surface should occur in much wider pores.
U.S. Pat. No. 4,216,188 relates to the preparation of pillared clay minerals from bentonite (montmorillonite). Here, polymeric hydroxy complexes of aluminum and chromium are mentioned as reagents for obtaining the pillars. The process of this patent distinguishes over that of U.S. Pat. No. 4,176,090 in that now the colloidal suspension of the starting clay mineral is prepared more carefully. The clay mineral is suspended in water and by treatment with NaCl the interlayer ions originally present are exchanged for sodium. Then the suspension is washed thoroughly and the last residues of NaCl are removed by dialyses. By centrifugation, the particles of less than 2 .mu.m are then separated. Next, the suspended clay particles are reacted with the polymeric aluminum or chromium complex, with the concentration of the chromium complex in particular being very low. After a thermal treatment at 150 to 450.degree. C. a BET surface of 160 to 240 m.sup.2 per gram is obtained. This patent mentions a distance between the elementary platelets of about 0.9 nm.
U.S. Pat. No. 4,248,739 describes a method wherein the pillars are provided using positively charged hydroxy complexes having a molecular weight of from 2000 to 20,000. However, the properties of the calcined pillared clay minerals are not significantly different from those mentioned in U.S. Pat. No. 4,176,090. The methods for the preparation of pillared clay minerals mentioned in U.S. Pat. No. 4,271,043 are not essentially different, either. Although the specification mentions that the thermal stability of the pillared clay minerals is high.
If form-selective catalytic reactions are to be carried out, a catalyst of narrowly defined pore dimensions is required. Zeolites satisfy this requirement exellently. However, a problem is that the transport in zeolites often proceeds poorly. Thus it has been demonstrated that molecules cannot pass each other in the pores of zeolites. Pillaring clay minerals also leads to pores of sharply limited dimensions, so that such materials can be suitable heterogeneous catalysts for such reactions. One condition, however, is that the materials can be prepared on a technical scale in a well reproducible manner.
Now, the preparation of suitable pillaring agents, such as the polymeric hydroxy complexes on a technical scale is difficult. In general, there are only a few businesses that produce suitable solutions of these complexes. In addition, in most cases the fraction of the aluminum that is present as Al.sub.13 is not large. As a consequence, very large volumes have to be employed to produce large amounts of pillared clay minerals, which in general is technically very difficult. Accordingly, one of the objects of the invention is to provide processes for the production of suitable pillaring agents on an industrial scale. Since the provision of the hydrated pillars in the clay mineral does not pose any problems technically, provided the clay particles are not too large, the production of the hydrated pillars on a technical scale seems to be the chief problem.
Purifying natural clays is cumbersome. In general, the clay must be suspended and the impurities allowed to settle. Then the clay must be separated from the suspension, which is technically problematic. This appeared hereinabove in the discussion of U.S. Pat. No. 4,176,090, which publication mentioned that much leakage of clay particles through the filter occurred. In addition, there is the problem that important clay minerals do not occur in nature or do so to an insufficient extent. One of the major problems in the use of natural clay minerals for catalytic purposes is moreover that although these materials may be very cheap, the properties are very difficult to control.
The synthesis of clay minerals according to the current state of the art is technically difficult. Customarily, a protracted (a few weeks) hydrothermal treatment is used at relatively high temperatures and pressures, under agitation of the aqueous suspension. In general, only a few grams or even only some tens of milligrams of a clay mineral can be synthesized simultaneously. The application of this technology on a large (industrial) scale is very difficult, if not impossible. As a result, synthetic clay minerals are costly. An example of such a synthesis, in this case of hectorite, is given in U.S. Pat. No. 3,666,407. Because hectorite has especially interesting rheological properties and does not occur much in nature, the synthesis of this mineral is of interest. This preparation starts from natural talc, which contains magnesium, oxygen and silicon and which occurs amply in nature in pure form. This material, after being crushed and mixed with lithium carbonate, is heated at 760 to 980.degree. C. for approximately 1 hour. After cooling, water glass and soda are added and for 8 to 16 hours the mixture thus obtained is treated hydrothermally, i.e. at high temperature and high pressure, under agitation of the preparation. It is clear that this is a relatively costly preparative procedure.
This also applies to the method for the preparation of synthetic clay minerals discussed in U.S. Pat. No. 3,671,190. In this patent it is observed that the preparative procedures thus far known have only been carried out on a laboratory scale and often yield only milligrams of the desired clay mineral which moreover is often polluted with quarts. In the method of U.S. Pat. No. 3,671,190, magnesium and silicon are coprecipitated by mixing water glass with a solution of a magnesium salt. The suspension thus obtained is then treated hydrothermally. To that end, the mixture is maintained under pressure at 250.degree. C. for about 4 hours with stirring. It is difficult to control the degree of crystallization and hence the dimensions of the crystallites.
Owing to the poorly controllable properties of natural clay minerals and the high price of synthetic clay minerals, the use of clay minerals for catalytic purposes has remained quite limited. Although the patent literature around 1980 evidenced much research effort in the field of the catalysis of (pillared) clay minerals, the technical application thereof has remained very slight.
Surprisingly, it has now been found that the above-mentioned tasks can be fulfilled by making use of clay minerals of which the dimensions of clay platelets are controllably variable from 1 .mu.m to 0.05 .mu.m, the stacking of elementary platelets can be controlled from on average one to three platelets to a number of approximately twenty platelets, while the ratio of different metal ions in the octahedron layer and/or tetrahedron layer is adjustable.